I've chased this issue this week. MLS briefs will generally cite a source for living area: "Owner", "Plans", "Appraisal", "Tax Records", etc. The brief for a comp cited "Tax Records" as the living area source; the record of the Assessor's office had the living area substantially less than the brief. I called the Assessor's office, which confirmed that the property had been measured. The Realtor said his source was from the planning/codes office, which takes that information from the plans builders provide when applying for the building permit.
The planning/codes people do not use the same criteria as the Assessors: they include covered and enclosed/screened porches in their area. In essence, there can be two (at least) public records sources for living area, and they may not be the same. And, it may not be surprising that MLS may reflect the larger area, whether knowingly or unknowingly. In this instance, the MLS/planning department living area for a comparable was about 11% larger than that shown by the Assessor. In developing this appraisal, I used the Assessor's area. (And, yes, it is less than that in MLS which, admittedly resulted in a smaller adjustment.) I understand that MLS data is relied upon by movers in the market, but to knowingly rely on and incorporate information known to be incorrect into an appraisal is, err, wrong.